This invention relates generally to electric motors and, more particularly, to a start winding cutout switch for a refrigerator compressor motor.
Electric motors typically include a start winding, a run winding and a magnetized rotor. The start winding is used to initiate rotation of the rotor. The run winding has a high inductive reactance relative to the start winding, so that the magnetic fields generated in the respective windings are out of phase with one another. The geometric time phase relationship between the magnetic fields causes the rotor to begin to rotate from a standstill condition when the windings are energized. Once the rotor has sufficient torque to attain its normal operating speed, the start winding is "cut-out" of the motor circuit so that the magnetic field generated by the start winding does not adversely affect motor operation. Alternatively, the start winding may be utilized as an auxiliary run winding after motor start-up by connecting a run capacitor in series with the start winding. Often, utilizing an auxiliary run winding results in better motor efficiency and power factor.
Low power current relays have been used to switch a start winding out of a motor circuit. However, the relays contacts are often short lived and susceptible to sticking together when switching the current, which would continuously energize the motor and cause burnout.
A positive temperature coefficient resistor (PTCR) may be used in lieu of a relay to regulate the current flowing through the motor start winding. A PTCR is a temperature responsive resistor element that has a low resistance in a cool state, and a very high resistance when heated to an "anomaly temperature" or "Curie Temperature." When a PTCR is connected in series with a start winding, the low initial resistance in the cool state allows the start winding to draw a relatively large current to accomplish initial motor rotation. As current flows through the PTCR, the current heats the PTCR, ultimately causing the PTCR to reach the Curie Temperature and the corresponding very high resistance state. Consequently, very little current flows into the start winding. Thus, the PTCR restricts or "chokes off" the current to the start winding to negligible levels. By selecting a PTCR so that the Curie Temperature is reached at approximately the same time when the motor running speed is achieved, a PTCR effectively regulates current flow into the start winding more reliably than a current relay.
A PTCR, however, consumes 2-3 watts of power to maintain the high resistance state at the Curie Temperature. In light of stringent energy consumption standards, PTCR energy consumption is a factor in the efficiency rating of a compressor motor. Therefore, energy savings could be realized, and efficiency ratings increased, by cutting the PTCR out of a circuit. While relay switches have been used in series with a PTCR to switch the PTCR out of the circuit, relay switches require power to keep the switch open, which affects the efficiency rating of the motor. Also, relay switches suffer from reliability problems with the switching contacts.
Accordingly, it would be desirable to provide a reliable cutout switch to remove a PTCR from a motor circuit. Further, it would be desirable to provide a cutout switch which does not consume power to keep the switch open.